There has been great interest to control and use light, or photons, in the same way that electrons are put to use in solids to make all types of electronic devices and provide communication between distant points. The development of fiber optics and semiconductor lasers have revolutionized the telecommunications industry. Optics is at the heart of the fabrication of integrated circuits, data storage, compact discs, and so forth. However, while the use of light has already demonstrated many advantages, there are considerable challenges ahead to fully make use of its potential. For example, it is desirable to engineer materials so that the propagation of light occurs only in certain directions for certain frequencies (i.e. photonic band gap materials). Efforts in these areas have led to translucent dielectric materials, known as photonic band gap materials or structures, which are opaque at certain frequencies. Light modes cannot propagate through the materials if their frequencies are within those defined by the band gap. The limitations of such materials is that they will permit light transmission at nearby frequencies. That is, these materials only block a narrow range of frequencies within a broad spectrum. Optical detectors are normally sensitive to a broad spectrum of light so that light of slightly different frequencies from that which is blocked might get through to the detectors and be detected. Therefore it would be much more useful to have a material or a device that operates exactly in a reverse manner, that is, it selectively transmits light only in a narrow range of frequencies within a broad spectrum.
The present invention has application in many areas, including those where the divergence of a light beam is a problem, or where increased light transmission through an aperture array is desirable, or in photolithography, or in optical filtering applications.
Opto-electronics, for example, is concerned with optical inter-chip communication in order to increase computation speed. The chips are usually all located on the same board and efforts are made to integrate optical lasers emitting from one chip to detectors located on other chips. One of the difficulties encountered is to make sure the emitted light does not diverge but rather remains collimated over sufficiently long distances so that the light reaches the desired detector or so that the light can be input into a fiberoptic bundle where each fiber is connected to a preselected chip or destination. As the structures and beam sizes approach that of the wavelength of light, the divergence and transmission of the light become even greater problems.
Also, conventional microscopes, and generally optical imaging and storage devices which operate in the far field, cannot resolve features substantially smaller than about one-half the wavelength of the light used. In order to overcome this resolution problem, near-field scanning optical microscopes (NSOM) were developed where an aperture much smaller than the wavelength of the probing light is placed near the specimen and scanned over its surface. A fraction of the light passing through the specimen is then collected through the aperture and relayed to a photodetector. Alternatively, light passes through the aperture, through the specimen and is then captured by a photodetector. The image of the specimen is then reconstructed by combining the signal at the photodetector with the microscope position over the specimen. However, the problem with an aperture smaller than the wavelength of light is that its transmissivity decreases rapidly and is proportional to the radius of the aperture divided by the wavelength to the power 4, i.e. (d/.lambda.).sup.4. As a result, much effort has gone into designing better apertures, such as fiber tips. However the transmission efficiency of these apertures is still orders of magnitude less than the optimal efficiency.
In another case, the resolution of photolithography, which is central to the chip manufacturing industry, is also limited in resolution to approximately one-half of the wavelength of the incident light. Techniques such as near-field scanning microscopy can be used to create smaller patterns in the photoresist, however such techniques are generally extremely slow since the photoresist patterns must be written on every chip. Unlike the case for traditional photolithography, the patterns cannot be projected through a mask, the standard industrial technique. In addition, as discussed above, the light transmission efficiency through smaller-than-wavelength apertures, such as tapered optical fibers, is very small. This slows the process even more because a minimum amount of light must impinge upon the photoresist in order to change its characteristics.
In another application, filters made from wire-mesh or metallic grids have been used extensively for filtering light in the far IR (infrared) spectrum (e.g. 10.about.800 micrometer wavelengths). These filters comprise thin metallic wires (much thinner than the wavelengths to be transmitted) deposited on an optically clear support. The filters are characterized by a transmission spectrum having a peak at approximately 1.2 times the periodicity of the mesh. The peak is very broad, typically greater than half the periodicity of the mesh. Mesh filters have been extensively studied and their properties have been explained by analogy with transmission line circuits. These filters would be much more useful if their transmission spectra could be narrowed to make them more selective.
The main object of the present invention is to overcome the problems and limitations described above by transmitting light very efficiently through an array of apertures, where each aperture is much smaller than one-half of the wavelength of the light and by allowing light transmission only at certain frequencies of light which can be controlled by the structure and arrangement of the aperture array.